(1) Field of the Invention
The present invention relates to a method of measuring low concentration of impurity in a high pure liquid such as high pure water, and a measuring apparatus therefor.
(2) Prior Art
Cleaning processes are employed in semiconductor manufacturing processes, especially in wafer processing of very large scale integrated (hereinafter referred to as VLSI) circuits. Such a process requires water of high purity. In case water includes a small amount of fine particles, microorganism, or non-volatile solute, such impurities substances come out on rinsed wafers, and they cause defects of minute patterns. Therefore, improvement of the purity of high pure water is required in order to improve yield of VLSI semiconductor devices.
In this connection, a measuring apparatus which can measure a concentration of particles or impurities in pure high water in high sensitivity has been required. One conventional measuring method includes a filtering step in which objective pure water is filtered by a filter, and a counting step in which number of particles on the filter is counted by an electron microscope. Another conventional method includes an evaporating step in which the objective water is evaporated and a measuring step in which impurity concentration of the water is measured by weighing dried remains.
However, since it takes a long time to measure the purity of pure water by such conventional methods, it is impossible to manufacture the pure water and to count the number of particles simultaneously. Furthermore, there is a shortcoming in that much manual skill is necessary.
Another conventional measuring apparatus is provided with a laser light source projected in the objective liquid, a light detecting means which detects light scattered by particles passing through a measuring region, and a signal processing means which counts photoelectric converted pulse trains. The apparatus can measure the number of particles or sizes. However, a high power laser light source such as argon ion laser is necessary for counting the number of fine particles by such a light scattering method. This results in the apparatus of large size. In addition, the range of particle size of these measurements is limited to e.g. above 0.1 .mu.m, and it is impossible to detect particles of smaller size, or impurities dissolved in a liquid.
Another conventional measuring apparatus is provided with an atomizer for atomizing an objective liquid by hot clean air and evaporating minute droplets, an optical means which irradiates floating impurities and detects scattered light, and a measuring means which measures both the number of particles and the particle sizes. However, the size range of this measurement is also limited to e.g. above 0.1 .mu.m. The lowest limitation of the measurement of concentration of impurities dissolved in high pure water is also about several 10 PPB (10.times.10.sup.-9). It is practically necessary to measure impurity concentration in high pure water lower than that value.